It is well known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal closure or container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators continually have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules, and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible, and more adhesive coating, but often improving one particular advantageous property is achieved at the expense of another advantageous property.
In addition, practical considerations limit the thickness, adhesive properties, and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing, and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal closure or container must be able to withstand the solvating properties of the packaged product. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products, such as beer, by imparting an off-taste to the product.
Organic solvent-based coating compositions provide cured coatings having excellent chemical resistance. Such solvent-based compositions include ingredients that are inherently water insoluble, and, thereby, effectively resist the solvating properties of water-based products packaged in the metal container.
Epoxy-based coatings and polyvinyl chloride-based coatings have been used to coat the interior of metal closures and containers for foods and beverages because these coatings exhibit an acceptable combination of adhesion, flexibility, chemical resistance, and corrosion inhibition. Polyvinyl chloride-based coatings and vinyl acetate/vinyl chloride copolymer-based (i.e., solution vinyl) coatings also have been the topcoat of choice for the interior of metal closures because these coatings provide excellent adhesion to plastisol sealer gaskets applied over the cured topcoat. However, epoxy-based coatings and polyvinyl chloride-based coatings have serious disadvantages that investigators still are attempting to overcome.
For example, polyvinyl chloride-based coating compositions are thermoplastic. Thermoplastic coatings used as the topcoat of the interior coating of metal closures have inherent performance disadvantages, such as potential softening during the closure manufacturing process or under food processing conditions. Therefore, coating compositions having a thermosetting character have been investigated.
In addition, coatings based on polyvinyl chloride or a related halide-containing vinyl polymer, like polyvinylidene chloride, possesses the above-listed advantageous properties of chemical resistance and corrosion inhibition, and are economical. However, curing a polyvinyl chloride or related halide-containing vinyl polymer can generate toxic monomers, such as vinyl chloride, a known carcinogen. In addition, the disposal of a halide-containing vinyl polymer, such as by incineration, also can generate toxic monomers. The generated vinyl chloride thereby poses a potential danger to workers in metal can and closure manufacturing plants, in food process and packaging plants, and at disposal sites. Disposal of polyvinyl chloride and related polymers also can produce carcinogenic dioxins and environmentally harmful hydrochloric acid.
Government regulators are acting to eliminate the use of polyvinyl chloride-based coating compositions that contact food, and thereby eliminate the environmental and health concerns associated with halide-containing vinyl polymers. Presently, however, polyvinyl chloride-based compositions are still used to coat the interior of food and beverage containers and closures.
To overcome the environmental concerns and performance problems associated with polyvinyl chloride-based coating compositions, epoxy-based coating compositions recently have been used to coat the interior of food and beverage containers. However, epoxy-based coatings also possess disadvantages. For example, epoxy-based coating compositions are more expensive than polyvinyl chloride-based coating compositions.
In addition, epoxy-based coatings are prepared from monomers such as bisphenol A and bisphenol A diglycidyl ether (BADGE), for example. ##STR1##
Epoxy resins have a serious disadvantage in that residual amounts of glycidyl ether and bisphenol monomers are present in the resin, typically in an amount of about 0.5% by weight. The presence of such monomers, and especially a glycidyl ether monomers, raises serious environmental and toxicological concerns, especially because a glycidyl ether monomer can be extracted from a cured coating on the interior of a metal container by a product stored in the container. Accordingly, regulatory agencies have promulgated regulations reducing the amount of a glycidyl ether monomer in coating compositions, and especially coating compositions used on the interior of food and beverage containers.
Coating compositions also typically include a phenolic resin. Phenolic resins prepared from bisphenol A or similar bisphenols also can contain residual bisphenol monomers, similar to epoxy-based coatings. Phenolic resins also have disadvantages in that the resins can generate formaldehyde, which can adversely affect a product stored in a coated metal container. Accordingly, it would be an advance in the art to overcome the problems and disadvantages associated with coating compositions for metal substrates that contain an epoxy resin, a halide-containing vinyl polymer, and/or a phenolic resin.
With respect to a metal closure for a food container, the interior of a metal closure conventionally can be coated with three separate coating compositions, i.e., a three-coat system. First, an epoxy/phenolic primer is applied to the metallic substrate and cured, then a vinyl-based middle coat is applied over the cured primer. Finally, after curing the middle coat, a specially formulated topcoat capable of adhering to a plastisol sealer is applied over the cured middle coat. The plastisol sealer is applied over the cured topcoat, and formed into a gasket during manufacture of a metal closure from a metal sheet having the three cured layers of coatings applied thereon.
Two-coat systems are the primary commercial system, but also exhibit disadvantages. Investigators are attempting to develop an improved two-coat system for coating the interior of a metal closure. An ideal two-coat system maintains corrosion inhibition, lowers the cost of applying the coatings, has improved rheological properties, has improved cured film integrity, is free of a polyvinyl chloride-based resin, residual bisphenol monomers, and residual glycidyl ether monomers. In addition, it would be desirable to provide a top coat that acts as a barrier against the migration of bisphenol and glycidyl ether monomers from an epoxy resin-based primer coat.
A two-coat system for the interior of metal food closure comprises a primer (i.e., a size) and a topcoat. The metal closures typically are used in conjunction with a glass or plastic container. The topcoat must have sufficient adhesion to the primer or the coating will fail. In order to achieve sufficient intercoat adhesion, the chemical makeup of the topcoat often was dictated by the chemical makeup of the primer. Investigators, therefore, have been seeking a more "universal" topcoat, i.e., a topcoat that can be applied to a variety of different primers and that exhibits sufficient intercoat adhesion. Such a universal topcoat would be a significant advance in the art.
The coatings used on the interior of a metal food closure also must meet other criteria in addition to performance. For example, the coatings must incorporate components acceptable to the U.S. Food and Drug Administration (FDA) because the cured coating composition contacts food products.
The cured primer and topcoat, therefore, require sufficient adhesion to maintain film integrity during closure fabrication. The cured primer and topcoat also require sufficient flexibility to withstand closure fabrication. Sufficient coating adhesion and flexibility also are needed for the closure to withstand processing conditions the closure is subject to during product packaging.
Other required performance features of the cured coatings include corrosion protection and adequate adhesion to the plastisol gasket applied over the cured topcoat. Also, the cured coating composition requires sufficient chemical resistance and sufficient abrasion and mar resistance.
In the manufacture of a metal closure, a metal sheet is coated with the coating compositions, and each coating is cured individually, then the metal sheet is formed into the shape of a metal closure. The closures are made in a variety of sizes ranging from 27 mm (millimeter) to 110 mm in diameter. During manufacture, a plastisol material is applied over the cured coatings on the interior of the metal closure. This plastisol subsequently is formed into a gasket and cured. The gasket ensures an effective seal between the metal closure and glass container, and to maintain the vacuum condition of the packaged food product.
Product packaging is performed under processing conditions wherein the plastisol gasket is softened. When the metal closure is pressed onto the glass container, the threads on the glass container form impressions in the softened plastisol gasket. The metal closure is secured in place both by the thread impressions and by the vacuum produced by subsequent cooling. This type of metal closure is used for baby food containers and for other packaged food and beverage products, such as juices and gravies. Other types of closures are designed to be secured to glass containers by lugs rather than by thread impressions in the plastisol.
Vinyl chloride-based topcoat compositions have been softened both by product processing conditions, and by conditions encountered during closure manufacture, thereby leading to closure failure. The present invention is directed, in part, to overcoming such closure failures.
Investigators, therefore, have sought a two-coat system for the interior of metal closures used for vacuum-packed food products. Investigators have particularly sought a vinyl halide-free topcoat for the interior of metal closures for food and beverages that retains the advantageous properties of a vinyl chloride-based topcoat, such as adhesion, flexibility, chemical resistance, corrosion inhibition, and favorable economics. Investigators especially have sought a coating composition that demonstrates these advantageous properties and also reduces the environmental and toxicological concerns associated with halide-containing vinyl polymers, formaldehyde, and residual glycidyl ether and bisphenol monomers.
Two-coat systems have been investigated and used for application to the interior of metal closures. Investigators sought and used topcoat compositions having a sufficiently flexible cured coating such that a coated metal substrate can be deformed without destroying film continuity. This is an important property because the metal substrate is coated prior to deforming, i.e., shaping, the metal substrate into a metal article, like a metal closure. Coating a metal substrate prior to shaping the metal substrate is the present standard industrial practice.
A present topcoat coating composition includes: (a) an acrylate copolymer having pendant glycidyl groups, typically a glycidyl (meth)acrylate-alkyl (meth)acrylic copolymer, and (b) an acid-terminated polyester, wherein the composition is free of a halide-containing vinyl polymer, and which, after curing, demonstrates: (1) excellent flexibility, (2) excellent adhesion to the primer coat, (3) excellent chemical resistance and corrosion inhibition, (4) excellent adhesion to the plastisol gasket, and (5) reduced environmental and toxicological concerns.
As an added advantage, a present topcoat coating composition is an improved two-coat system, thereby eliminating the presence of a halide-containing vinyl polymer and the presence of residual bisphenol and glycidyl ether monomers, while providing an effective barrier against migration of residual bisphenol and glycidyl ether monomers from the size coat. The present topcoat coating composition also can be used with a variety of types of primers without a significant decrease in coating properties.